- 學位論文
利用計算流體力學分析與模擬超臨界水之熱傳劣化現象
Simulation of Forced Convective Heat Transfer of Super Critical Water Using the Technic of Computational Fluid Dynamics
摘要
超臨界水反應器為第四代核反應器中的一種。其以高熱效率(45%)，以及簡化的系統著稱。然而，由於超臨界水的特殊熱力性質，超臨界水反應器的設計需掌握兩種知識：(a)超臨界水的熱傳現象 (b)超臨界水熱傳係數的預測。超臨界水反應器操作於極大壓力之下(25 MPa)，做實驗相當昂貴，因此本論文以商業計算流體力學(Computational Fluid Dynamics, CFD) FLUENT軟體來探討超臨界水的熱傳現象。 強制對流的熱傳分析，當流動達到紊流時，一般使用最廣的經驗式為Dittus-Boelter關係式。當量測的熱傳遞係數比用Dittus-Boelter關係式所得的值還大時，稱為熱傳強化現象;反之，當量測而得的熱傳遞係數比用Dittus-Boelter關係式所得的值還小時，稱為熱傳劣化現象。根據模擬結果，熱傳強化現象已可利用RNG紊流模型準確模擬。最大的熱傳遞係數發生於壁溫稍低於準臨界溫度的地方是因為近壁區比流體中心區域更早達到比熱最大值。熱傳劣化的模擬，雖然可看到劣化現象，但與實驗數據仍有段差距。最後考慮了實驗的不準確度，探討各種不同進口溫度與操作壓力對壁溫分佈的影響，發現，操作壓力的增加與進口溫度的提升，都可有效的減緩熱傳劣化的發生。 最後，分別探討了浮力作用對於熱傳強化與惡化的影響。模擬結果顯示浮力作用在熱傳強化中幾乎可以忽略，但卻是造成熱傳劣化的主要原因。
並列摘要
Supercritical Water Reactor (SCWR) is one of the Gen IV systems. SCWR has the advantages of high thermal efficiency (45%) and highly simplified plant systems. Due to the large variations of thermal properties of supercritical water when the temperature is close to the critical temperature or pseudo critical temperature , there are two things of concern for the design of supercritical water reactor. These are the heat transfer phenomenon of supercritical water and the prediction of heat transfer coefficient. SCWR operate at very high pressure (25 MPa) , therefore ; it is very expensive to study heat transfer phenomenon of supercritical water experimentally. Commercial Computational Fluid Dynamics (CFD) software FLUENT is used to study the heat transfer of supercritical water. Usually, Dittus-Boelter correlation is used to predict heat transfer coefficient of forced convection. If the heat transfer coefficient as predicted by the Dittus-Boelter correlation is higher than the experimental data, the situation is termed heat transfer deterioration. If the heat transfer coefficient as predicted by the Dittus-Boelter correlation is lower than the experimental data, the situation is termed heat transfer enhancement. The results of simulation demonstrated that heat transfer enhancement can be accurately predicted using the RNG turbulence model. Maximum heat transfer coefficient is occurred before the bulk temperature reaches the pseudo-critical temperature because the wall temperature reaches the pseudo-critical temperature earlier than the bulk coolant temperature does. The results of the analyses show that the phenomena of heat transfer deterioration can be simulated. Nevertheless, the locations of the heat transfer deterioration as predicted by the FLUENT code are deviated from the experimental data of Shitsman’s. It is also found that the phenomenon of heat transfer deterioration can be avoided by increasing the inlet temperature or operation pressure. The results of simulation demonstrated that the buoyancy has no effect on heat transfer enhancement. Nevertheless, it is the main reason of the heat transfer deterioration.